There is no way that it should be getting less than 25 shots if it is chrono'd at 280.
I'd suspect the valve to be kept open by a weak valve spring if there is absolutely no leaks. I wouldn't rule out a leak as they can be very small.
Most of the time I have a leak it's from not screwing the co2 cap in enough.
Once you get it working.... It's possibly the most fun gun I've ever had.